5' sequences have been determined for poliovirus type 1 (PV1), poliovirus type 2 (PV2) and Coxsackie virus B1 (Cox B1), using direct sequencing of 5' labeled molecules. Results demonstrate that the 5' region of these enteroviruses is conserved. For instance PV1 and PV2 have the identical first 17 nucleotides. PV1 and Cox B1 are identical up to nucleotide 10, after which the sequences diverge. PV1 and Cox B1 have less than 5% overall sequence homology. Thus, the common 5' sequence has been retained in spite of great sequence divergence. Experiments with a temperature sensitive mutant of PV1, tsB9, have concentrated on establishing the relationship of the defect to the molecular events during infection. We have observed two anomolies when tsB9 infects HeLa cells at 39.5 degrees C. First, the mutant fails to make 35S RNA, and, instead produces a large replicative-like structure. Secondly, tsB9-infected cells at 39.5 degrees C have no procapsid structures, although viral protein synthesis is at near normal levels. We are examining these two properties of the mutant in an effort to understand aspects of poliovirus replication.